There is a recent USB (Universal Serial Bus) device operated by supplying a power source from a personal computer (computer) at the time of being connected to the personal computer. That is, there is a USB device operated by bus power. Such a USB device is disclosed for example in Patent Document 1 and Patent Document 2.
A USB device disclosed in Patent Document 1 is provided with two signal wires used for inputting and outputting data, a 5-V power wire, and a ground (GND) wire. By providing a switch for switching the connection and cut-off of the 5-V power wire, a power source supplied to the USB device is turned ON/OFF.
According to Patent Document 2, electric power capable of being supplied by the bus power is regulated to 5 V and 500 mA. The USB device is provided with a secondary battery for storing electric power supplied from a personal computer. When an electric current flows exceeding an upper limit of 500 mA in such a USB device, a shortage is supplied by the stored electric power.